Chemoprevention of prostate carcinogenesis is a plausible and necessary approach to deal with the prostate cancer (PCa) problem at the root. Previous studies have suggested that supplementation of selenium (Se) may prevent or delay human PCa. However, the NCI stopped the Selenium and vitamin E Cancer prevention Trial (SELECT) in October 2008 ahead of schedule because of the failure to demonstrate an efficacy of selenomethionine (SeMet) for PCa prevention in North American men and a slight but non- statistically significant increase in diabetes risk. Because of the metabolic and biochemical differences between SeMet and other Se forms, the failure of SeMet in the SELECT study should and cannot be equated to all Se forms as ineffective for PCa prevention. We have compelling evidence that daily oral supplementation of putative precursors of methylselenol, methylseleninic acid (MSeA) and Se-methylselenocysteine (MSeC), inhibit the in vivo growth of human PCa xenograft in athymic nude mice whereas SeMet does not; and that MSeA and MSeC inhibit primary carcinogenesis in the transgenic adenocarcinoma mouse prostate (TRAMP) model with significant survival benefit. While these in vivo data suggest these second-generation Se compounds (vs. SeMet) as promising bioactive supplements for PCa prevention and merit consideration for future translational studies, inherent limitations of the TRAMP model temper the value of these data for human translatability. Therefore, in this R21 proposal in response to PA-07-362, we hypothesize that MSeA, as a representative of second-generation Se, inhibits/prevents PCa progression from PIN to adenocarcinoma through targeting Akt signaling pathway in a Pten-deficiency mouse model. Two specific aims are proposed. Aim 1: To determine the efficacy of MSeA in preventing Pten deletion-driven PCa development in vivo. We will generate prostate-specific Pten-deletion (Pten-/-) mice using Cre-loxP technique. The wild type Pten+/+ mice and Pten-/- mice will be treated with 2 dosages of MSeA at 7 weeks of age (WOA). At 12 WOA and 27 WOA, 10 mice of each group will be sacrificed. An extensive histopathology survey will determine the early lesion responses. Tissue samples will be banked for verifying molecular targeting (Aim2). Additional cohorts (n=20) of Pten-/- mice will be used to assess the beneficial impact of MSeA on their survival, PCa burden (weight, number) and metastasis. Aim 2: To test whether the preventive efficacy of MSeA is associated with a suppression of AKT signaling pathway. We will measure AKT phosphorylation and its downstream targets by immunohistochemistry and Western Blot in prostate tissues collected at the first 2 endpoints and correlate them to indices of cell proliferation and cell death and overall survival benefit. Impacts of the results: The results will provide proof-of-principle data regarding the usefulness of Pten-/- model for assessing MSeA as a bioactive dietary supplement in vivo and lay the ground work for future R01/P01 projects for developing this and other second-generation Se for molecular pathway-targeted PCa prevention. PUBLIC HEALTH RELEVANCE: The ill-reputed selenomethionine in the Selenium and vitamin E Cancer prevention Trial (SELECT) study has brought a lot of negative publicities on the whole research field of selenium-cancer prevention and treatment. Rigorous studies as proposed here that use a clinically relevant pre-clinical model and a judicious choice of selenium agents with compelling mechanistic rationale and support (unlike the case with SeMet for SELECT) are essential and necessary to produce relevant data to support continued work to realize the full health benefits of the second-generation selenium and change perceptions and understanding of scientists and the public alike on selenium's merits.